1. Field of the Invention
The present invention relates to electrostatic holding devices and more particularly to devices for holding semiconductor wafers and similar materials during microelectronic device manufacturing operations such as electron beam exposure, etching, polishing, sputtering, annealing, vapor deposition and the like.
2. Description of the Related Art
The processing of semiconductor wafers into useful microelectronic devices requires a level, horizontal clamping of the wafers inside various processing machines, Wafers themselves, or patterns on an upper surface of such wafers can be easily damaged by chipping which result in dirt particles adhering onto the patterned surface or wafer breakage. Many wafer clamping methods are known such as those which use mechanical clamps or electrostatic forces to attach the wafer to a support. An example of a mechanical clamp is shown in the U.S. Pat. No 4,603,466 where an outer ring presses down on the front face of a wafer, while a domed back surface is pressurized.
However, wafer stress and possible damage to the wafer surface result from such an arrangement. Electrostatic methods of holding semiconductor wafers are preferred due to the ability of electrostatic clamps to operate without contacting the delicate front surface of patterned semiconductor wafers. One electrostatic chuck is described in U.S. Pat. No. 4,412,133 wherein electrostatic forces are used in addition to that of an outer mechanical clamping ring. A method of enhanced electrostatic attraction is described in U.S. Pat. No. 4,554,611 by utilizing charge retention, mechanically jiggling the wafer and then disconnecting the applied voltage. Other electrostatic chucks are exemplified by U.S. Pat. Nos. 3,983,401; 5,325,261 and 5,103,367.
Electrostatic chucks typically comprises a flat platform through which a relatively large static electric field is transmitted. The platform supports a wafer which has been placed thereon, and a thereafter applied electrostatic field firmly holds the wafer in place and prevents its movement during treatment processes. When the desired treatment processes are completed, the electric field is reduced and the wafer removed from the platform surface.
One problem with known electrostatic chucks is that the platform tends to become damaged after being subjected to microelectronic device manufacturing operations such as electron beam exposure, etching, polishing, sputtering, annealing and vapor deposition. This damage has usually necessitated a complete replacement of the platform. An improved platform which is known in the art employed a support having a raised central plateau. The edge of the platform was coated with a dielectric paint and the plateau applied with a replaceable dielectric sheet. Thus if the sheet becomes damaged, only the sheet is replaced rather than the entire platform. A problem with this arrangement is that an sharp edge is formed between the raised central plateau and the region surrounding the plateau. A much higher localized electric field is produced at this edge which causes arcing and breaking through the dielectric sheet. The present invention solves this arcing problem by providing a central plateau which does not have a sharp edge, but rather has an edge which tapers down to the region surrounding the plateau. A dielectric coating or paint is then applied on the taper and surrounding region such that the top level of the coating is level with the top of the plateau without covering the central part of the plateau. The dielectric sheet is then removably attached, for example by an adhesive, to the central part of the plateau, and to the coating on the taper and surrounding region. The platform is thus both protected by the dielectric sheet and arcing is prevented.